Fluorescent lamps are used to provide illumination for general lighting purposes, including backlighting for Liquid Crystal Displays (LCDs). The critical factors in the design of a cold-cathode fluorescent lamp (CCFL) include efficiency, cost, and size. A fluorescent lamp is a low-pressure gas discharge source. The lamp contains mercury vapor at low pressure with a small amount of inert gas. The inner wall of the lamp is coated with fluorescent powder. The discharge generates visible radiation. The CCFL has efficiency in the range of 40 to 60 lumens per watt and the average life of the CCFL lasts for 10,000 hours or more.
In a typical lighting application, it may be desirable to monitor input current or other characteristics to maintain consistent brightness. This may include implementing a feedback loop. A CCFL is effectively a constant voltage device. For example, the voltage across a typical CCFL may be about 600–700V rms. If connected in parallel, some CCFLs would hog most of the current, based upon placement and lamp-to-lamp production-variation characteristics. So CCFLs are difficult to mount in parallel and typically require a separate output circuit for each lamp.
Despite this problem, efforts have been made to incorporate multiple CCFLs in single controller topologies. For example, a current balancing scheme for multiple CCFLs is shown in U.S. Pat. No. 6,459,216, which issued Oct. 1, 2002, to Ying Hsien Tsai, entitled “Multiple CCFL current balancing scheme for single controller topologies”, which is incorporated by reference.
External Electrode Fluorescent Lamp (EEFL) technology is relatively new. Unlike with CCFLs, which have an electrode on each end of the lamp located inside the tube in direct contact with the plasma, EEFLs have electrodes that are located outside the tube. An EEFL makes contact on the outside of a foil strip, which forms a capacitor between the foil and the plasma. Thus, the electrode at each end does not actually touch plasma inside the tube. This arrangement results in a lamp that does not generate heat and increases lifetime and illumination stability, and decreases power consumption.
Advantageously, EEFLs can easily be configured in a ‘parallel’ design. EEFLs can be driven in parallel because most of the voltage is across the capacitors, which causes the current to be shared between the lamps. In other words, the capacitors act as a ballast that causes the current to be shared between the lamps. It is therefore possible to run banks of 2, 5, 20, or even more EEFLs from a single suitable inverter. The number of EEFLs in a bank may grow, as well as the size of each EEFL, as the technology improves.
Unfortunately, for an EEFL at 700V rms inside the tube, the voltage outside may be as high as 2000V rms, which means a relatively large amount of voltage is dropped across the capacitors. Thus, there is no “cold end” to the lamp because voltages are too high. This is significant because, for example, feedback circuits used with CCFLs typically rely upon the CCFL including a “cold end,” which is run to ground. Thus, prior art feedback circuits have proven difficult to implement with EEFL banks.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.